A few months ago, we showed you a dos-à-dos book--one with a hard back that forms the front of another book. This rare book owned by the National Library of Sweden is even more complex. Erik Kwakkel, a medieval book historian at Leiden University, says that this book is actually six books that are each opened differently. Each book opens and closes with a little clasp.

All of the books are devotional texts printed in Germany in the 1550s through 1570s, including a copy of Martin Luther's widely-read The Shorter Catechism. You can view more photos here.